Constant envelope RF transmitters are employed in some communications applications. Such transmitters may employ a RFPA to which power tuning is applied wherein the gain is adjusted according to the RF power level of the signal being transmitted. This is usually done as follows. The RFPA is in a circuit which includes means for applying to the RFPA a supply voltage and also means for applying to the RFPA a gain control bias voltage. The latter means includes a feedback control loop which includes a sampler for sampling an output of the RFPA, a controller comprising (a) a RF detector for detecting a level of RF power sampled by the sampler; (b) a comparator for comparing an output signal from the RF detector with a reference signal; and (c) an integrator for integrating an output of the comparator.
Using such a known RFPA circuit, a constant level of modulated carrier is applied at the RFPA input terminal. The sampler, which may for example comprise a directional coupler, samples the signal at the RFPA output. The RF detector of the controller detects the level of power sampled. The comparator then finds the difference between the level of sampled RFPA power and the reference signal and the result is integrated by the integrator. The signal produced as an output by the integrator is applied as a gain control input voltage to the RFPA at a suitable control input terminal.
A memory associated with the transmitter stores a table of values of the appropriate level of reference signal corresponding to specific values of RF output power level. The table is initially constructed by use of theory and/or experimentation.
There is a major disadvantage with the above known approach. For such a power control arrangement, the RFPA needs to work near its 1-2 dB compression point and not in saturation. If the RFPA is arranged to work in saturation its small signal gain and thus the control loop small signal gain will drop and the control loop will be effectively open. The control loop will not work under such conditions. However, working near the 1-2 dB compression point is not ideal either because the RFPA efficiency is not as good as in saturation. The efficiency decreases as the working point is further from the saturation point (Psat).
Another disadvantage of the above known approach is that the RFPA efficiency will depend on the individual RFPA saturation point and may vary as a result in a given batch of RFPAs. For example, assume that it is required to transmit a power level of 30 dBm and that Psat of a given batch of RFPAs has a statistical distribution of between say 31.5 and 32.5 dBm. It is clear that for a RFPA with Psat of 32.5 dBm the efficiency will be worse than for a RFPA from the same batch with Psat of 31.5 dBm.
The purpose of the present invention is to provide an improved RFPA power control circuit for a RF transmitter, particularly for transmitting a constant power envelope, wherein the above disadvantages are reduced or eliminated.